Le sujet du mail. -- The subject of the mail (of a specific mail)

Le sujet de mail. -- The mail-subject (in general)

So, in the same writing challenge, "Master d'Anglais" is general? I have a lot of trouble seeing what is general in French. To me, it might have been Masterd'autre chose, so it seems specific. In another spot they use "entourer de falaises." Since we are describing a specific area and specific cliffs, why not entourer des falises? 

In some cases it is easier to see than in others. 

Une chanson d'amour. -- a love song (a song about love in general). 

Une chanson de l'amour. -- a song about love. 

I would say that without the article, the second noun modofies the first noun. So you could say:

Une chanson d'amour est une chanson de l'amour. 

The input of a native speaker would be appreciated.